Traditional approaches to generating aerosol often do not use heat energy, and include pressurised containers with valves, dry powder formulations or nebulisers.
Electronic cigarettes are relatively well known in the art and are becoming popular as a substitute for a regular cigarette. The most common operating principle for such electronic cigarettes is to provide a flavoured aerosol to a user without burning material. Such an aerosol can comprise inhalable mixtures of gases, liquid aerosols (liquid particles suspended in air) and/or solid aerosols (solid particles suspended in air). One of the most common approaches to providing the flavoured aerosol is to have a reservoir of aerosol-generating liquid which is vaporised by provision of an electrically powered heat source to which the liquid is provided.
Aerosol-generating liquids typically contain, in varying proportions, at least one aerosol producer (often propylene glycol and/or glycerol), diluents (water and/or ethanol), flavours and/or tobacco extracts, and aroma compounds.
Some prior art electronic cigarettes operate through a user depressing a switch which activates the heating process after which they can inhale from the electronic cigarette. Others have an activation switch for the heater which is operated when a user inhales and draws air through the electronic cigarette.
Whilst such electronic cigarettes are becoming popular, consumers seem to experience some problems with them. For example, it can be difficult for the user to control the level of intensity or strength of flavour that is delivered to them. Attempts have been made to overcome this by having a control over the element that creates the aerosol, but such devices often provide such low level of aerosol at some settings that consumers dislike them.
Furthermore, some compounds in the aerosol-generating liquid that are used to provide flavour to the aerosol can lose aroma character over time. As an example, vanillin, a vanilla flavour component, reacts with propylene glycol when mixed, reducing the flavour generated by the vanillin and also producing a deep red discolouration in the aerosol-generating liquid.
The present invention seeks to provide an aerosol generating device which overcomes at least some of the above problems.
According to the present invention there is provided an aerosol generating device comprising:                at least two reservoirs, each arranged to contain a separate aerosol producing composition;        means for drawing the compositions from the reservoirs to generate an aerosol for inhalation for a user, and        means for mixing the compositions after they have been drawn from the reservoirs, the mixing means being arranged to allow a user to select the relative proportions of the two compounds in the aerosol.        
In a preferred embodiment of the invention the aerosol generating device is an electronic cigarette and comprises:                a power source;        at least one heater assembly;        a means for selectively applying power from the power source to the heater assembly;        at least two liquid-containing reservoirs, each arranged to contain a separate aerosol producing composition; and        control means for selectively controlling the heating of liquid from one or more of the reservoirs by the heater assembly, such that, when power is supplied to the heater assembly together with the selected liquid, an aerosol is generated for inhalation by a user.        
With the present invention, it is possible for a user to control the content of the aerosol that they receive by controlling the source of the liquid.
This control may also allow the mixing of components during aerosol generation such that they are not mixed in advance and therefore not subject to adverse aging effects.